Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a probe or “sonde” is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.
In these and other logging environments, it is desirable to construct an image of the borehole wall. Among other things, such images reveal the fine-scale structure of the penetrated formations. The fine-scale structure includes stratifications such as shale/sand sequences, fractures, and non-homogeneities caused by irregular cementation and variations in pore size. Orientations of fractures and strata can also be identified, enabling more accurate reservoir flow modeling.
Borehole wall imaging can be accomplished in a number of ways, but micro-resistivity tools have proven to be effective for this purpose. Micro-resistivity tools measure borehole surface resistivity on a fine scale. The resistivity measurements can be converted into pixel intensity values to obtain a borehole wall image. However, oil-based muds can inhibit such measurements due to the variability of impedance in the mud surrounding the tool. U.S. Pat. No. 6,191,588 (Chen) discloses an imaging tool for use in oil-based muds. Chen's resistivity tool employs at least two pairs of voltage electrodes positioned on a non-conductive surface between a current source electrode and a current return electrode. At least in theory, the separation of voltage and current electrodes eliminates the oil-based mud's effect on voltage electrode measurements, enabling at least qualitative measurements of formation resistivity.
In constructing an imaging tool for use in oil-based muds, certain engineering constraints on the structural strength of sensor pads will be recognized. The engineering constraints may be met by making the sensor pad base out of a metal such as steel. Though the steel can be insulated to present a non-conductive external surface, the electrical conductivity of the base creates potential current leakage paths via the metal body of the pad. These leakage paths affect the accuracy and stability of the tool's resistivity measurements and can cause error in the measurement of formation resistivity, especially when the source current operating frequency increases.
Another source of formation resistivity measurement error is caused by the finite input impedance of the differential voltage amplifier circuitry coupled to the differential voltage sensing voltage electrodes. This error is further exacerbated by the presence of a common mode voltage between the formation under the voltage electrodes and the reference voltage of the amplifier circuitry. The finite input impedance of the amplifier circuit allows a small amount of current to flow into the voltage electrodes and amplifier, creating a variable voltage divider that causes the common mode voltage to affect the differential voltage at the voltage electrodes. The influence of the common mode voltage on the differential voltage measurement creates inaccuracies in the borehole resistivity images.
One proposed method of reducing the common mode voltage relies on isolating the current source transmitter circuitry from the reference ground of the amplifier. For this method to work, the impedance of the isolation between the transmitter and the reference ground would have to be significantly higher than the impedance between the voltage electrodes and the formation. Unfortunately, such an environment would be very difficult, if not impossible, to achieve because the impedance from the voltage electrodes to the formation is often much higher than the parasitic impedance from the transmitter to the amplifier reference ground due to the presence of the layer of oil-based mud in the borehole. Accordingly, an improved method and system to minimize the effects of a common mode voltage signal in borehole resistivity imaging is needed.
The drawings show illustrative invention embodiments that will be described in detail. However, the description and accompanying drawings are not intended to limit the invention to the illustrative embodiments, but to the contrary, the intention is to disclose and protect all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.